Democrats Will Hope for a ‘Bump’ at Next Week’s Debate. What’s It Worth?

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After next week’s Democratic debate, pundits will inevitably argue over who won and who lost, and they’ll scour the polls to see if any presidential candidate has gotten a “bump” in support.

But as high-profile as they are, debates rarely help a candidate break out of a crowded field. And strategists generally agree that post-debate spikes in the polls seldom last for long.

A dozen candidates will be onstage at the CNN/New York Times debate on Tuesday, but experts broadly agree that none of them are likely to see a significant change in fortunes in a single night.

Ahead of the 2016 Republican primaries, Donald J. Trump’s publicity-heavy campaign rolled along with impregnable strength, despite the fact that TV viewers rarely gave him high marks for his debate performances.

The current Democratic race has been characterized by a similar steadiness, experts say, with former Vice President Joseph R. Biden Jr. continuing to command the support of around a quarter of Democratic respondents, and Senator Elizabeth Warren of Massachusetts rising consistently and sometimes surpassing him.

“Democratic primary voters basically seem to be fine with their top three options,” said Sean McElwee, a co-founder of the polling and analytics firm Data for Progress, referring to Mr. Biden, Ms. Warren and Senator Bernie Sanders of Vermont.

“There’s the possibility that a debate would drive a news cycle that would bolster a candidate,” Mr. McElwee added. “But instead, what we’ve seen is that those sorts of moments tend to fall off.”

Senator Kamala Harris of California is the only Democratic candidate this cycle who has seen a statistically significant jolt in the polls after a debate. At the first round of Democratic debates in late June, she confronted Mr. Biden on his past opposition to federally mandated school busing. She immediately leapt up in the polls, enjoying double-digit support among primary voters nationwide throughout July.

Perhaps more important, the exchange gave her fund-raising operation a shot of adrenaline: Just 24 hours after the debate, she had raised $2 million.

But she soon fell from the race’s top tier, and has not broken into the double digits in any major national poll since the start of August.

Mr. McElwee cited recent studies showing that candidates’ success is strongly driven by the sheer amount of news coverage they manage to receive — an argument that was borne out by the success of Mr. Trump in the run-up to the 2016 primaries, when he was able to reverse the unfavorable opinions that most Republican voters held of him, not by changing the tenor of his campaign, but by securing almost nonstop coverage.

“There are lots of different earned-media opportunities in the current climate — and debates are very important,” Mr. McElwee said. “But if you look at the polling, there’s been lots of people who have had good debates, bad debates, and at the end of the day, it hasn’t really changed the broad structure of the race.”

“In the broad scope of cable news coverage, the one hit per month of the debate is not enough to make a candidate break through,” he added.

The Democratic pollster Celinda Lake said that debates can help drive future coverage, resulting in a snowball effect. But she added that candidates’ fortunes will largely be determined on the ground in early-voting states.

“I think that honestly, the debates are having more impact on news coverage than they are having impact on voters,” Ms. Lake said. “Candidates don’t get known by unveiling a good plan at the debate. They become known by immersing themselves in these early states.”

To qualify for November’s debate, candidates will need to reach 5 percent support in two polls in early nominating states, or 3 percent in four early-state or national polls. These are higher thresholds than for any debate thus far. A bump after next week’s debate could, at the very least, help candidates such as Senator Amy Klobuchar of Minnesota or the former housing secretary Julián Castro — who are now foundering in the low single digits — secure a spot onstage next month.

But historically, early debates have been more likely to disqualify a candidate than to provide a lasting jolt of momentum. In 2011, Gov. Rick Perry of Texas pledged in an early debate to eliminate three agencies from the executive branch if elected president but couldn’t remember the name of one of the agencies. He never recovered from the flub. (When Mr. Trump became president, however, Mr. Perry was chosen to head the Department of Energy, the very agency he had forgotten to name.)

Last election cycle, the businesswoman Carly Fiorina enjoyed a strong showing in the second Republican primary debate, seeing a sizable uptick in support in the days afterward. But she could not convert it into a consistent climb in the polls.

“Successful first debate performances create buzz for a candidate but also generate expectations for continued success,” the historian and presidential scholar Bruce Buchanan wrote in an email. “If he or she does O.K. but less well in the second debate, the story is about underperformance.”

Bob Shrum, a Democratic strategist and speechwriter, said that Ms. Warren had found success by performing strongly in debates — while not chasing a breakout moment. “She has done this by performing consistently well by not looking for a cute line, and my guess is that she’ll continue that,” he said.

But Mr. Shrum noted that as the field of candidates narrows, and the remaining candidates each get more airtime, the opportunity for news-making moments will increase. “These early debates are more like skirmishes,” he said. “We’re now getting into the later debates — and they can have an impact.”

It may not be until the general election that the debate stage once again assumes prime importance.

“Primary debates are more like a beauty contest or a cameo appearance in a play,” Peter Hart, a longtime Democratic pollster, said. “It gives you a glimpse, it gives you an insight, and it allows a candidate to get noticed. More importantly, it can eliminate a candidate’s potential to be the nominee.”

On the other hand, he said, “general election debates have a huge impact upon the dynamics of the election — and the outcome.”